In phosphate complexes of the type for manufacture of wet process phosphoric acid and sometimes also for manufacture of phosphate chemicals utilizing phosphoric acid in their manufacture, water is employed as a coolant for gas streams created within the complex. The cooling water absorbs and dissolves various materials, and is sent to a pond for cooling. Typical pond water will have a pH in the range from about 1.5 to about 2.0, more or less, typical cooling pond water compositions and characteristic being shown in Table 1.
TABLE 1 ______________________________________ TYPICAL COOLING POND WATER CHARACTERISTICS Parameter Range* ______________________________________ pH, Standard Units 1.8-2.1 Total Acidity, as CaCO.sub.3 20,000-40,000 Fluoride, as F 4,000-8,000 Phosphorus, as P 4,000-8,000 Silicon, as Si 1,000-3,000 Total Solids 20,000-40,000 Total Suspended Solids 50-250 Conductivity, umhos 15,000-30,000 Chlorides, as Cl 50-250 Sulfates, as SO.sub.4 2,000-8,000 Sodium, as Na 50-3,000 Calcium, as Ca 50-150 Magnesium, as Mg 50-300 Aluminum, as Al 50-400 Chrome, as Cr 0.2-2.0 Zinc, as Zn 1.0-5.0 Iron, as Fe 100-250 Manganese, as Mn 5-30 NH.sub.3 --N, as N 0-1,200 Total Organic N, as N 3-30 Color, APHA units 1,000-4,000 ______________________________________ *All values expressed as mg/1 unless otherwise noted
Various federal and state agencies have established limits for fluoride and phosphorus compositions in waste waters to be disposed of. The limits vary somewhat from area to area, but the average limits are about 25 mg/l for fluorides and about 35 mg/l for phosphorus. Water containing an excess of either or both of fluorides and phosphorus may not be disposed of either into surface waters or into underground water disposal areas.
To remove fluoride and phosphorus components of waste cooling pond water, it is necessary to neutralize the water in order to precipitate fluoride and phosphate salts. Methods have been proposed and used wherein limestone and lime are used in a first neutralization stage, and lime alone is used in a second neutralization stage. These methods were described in a presentation by G. A. Mooney, A. T. Nogueira, and C. G. Thompson at the 1977 joint meeting of the Central Florida and Peninsular Florida sections of the American Institute of Chemical Engineers, May 20-22, 1977, the presentation being contained in a printed report of the presentation. While these methods reduce the fluoride and phosphate compositions of the waters to satisfactorily low levels, there are precipitates created which are disposed of in settling ponds, at least one for each stage of neutralization. This invention seeks to provide improved methods for neutralizing waste pond waters from wet process phosphoric acid manufacture and to simplify operation of plants incorporating the same, by eliminating conventional settling ponds for disposing of precipitants resulting from the neutralization by disposing of the precipitates in a gypsum stack, and from fully meeting the requirements of federal and state regulations. Conventional settling ponds, at least one for each stage of neutralization, eventually fill with solids and require periodic cleaning at great expense. Large land areas, five to twenty acres, are required for these ponds but are not generally available at many operational sites at reasonable cost or in close proximity to operations.